Mark server-live sidecar passes as live mode and cover the stream heartbeat with a test

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/server/transcribe.ts

@@ -316,6 +316,8 @@ export interface TranscribeDeps {
   maxUploadBytes?: number;
   /** Directory for uploaded-PCM temp files. Default: the OS temp dir. */
   tempDir?: string;
+  /** Blank-line heartbeat period on the ndjson stream (D-021). Default 5 s. */
+  heartbeatIntervalMs?: number;
 }
 
 function json(body: unknown, status: number): Response {
@@ -409,6 +411,7 @@ async function writeBodyToFile(
 export function createTranscribeHandlers(deps: TranscribeDeps): TranscribeHandlers {
   const maxUploadBytes = deps.maxUploadBytes ?? DEFAULT_MAX_UPLOAD_BYTES;
   const tempDir = deps.tempDir ?? tmpdir();
+  const heartbeatIntervalMs = deps.heartbeatIntervalMs ?? HEARTBEAT_INTERVAL_MS;
 
   return {
     async status(request) {
@@ -460,7 +463,7 @@ export function createTranscribeHandlers(deps: TranscribeDeps): TranscribeHandle
             } catch {
               clearInterval(heartbeat);
             }
-          }, HEARTBEAT_INTERVAL_MS);
+          }, heartbeatIntervalMs);
           const chunks: TranscriptChunk[] = [];
           try {
             await deps.sidecar.transcribe(